Make remaining methods const #2881

Job	Run time
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	17s
features_check (ubuntu-latest)	5m 15s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	10s
no_std (ubuntu-latest, thumbv6m-none-eabi)	12s
features_check (macos-latest)	11m 34s
alternative_targets (ubuntu-latest, aarch64-linux-android)	24s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	11s
features_check (windows-latest)	9m 53s
rust_msrv (ubuntu-latest)	1m 23s
rust_versions (ubuntu-latest, stable)	1m 10s
rust_versions (ubuntu-latest, beta)	1m 1s
timezones (ubuntu-latest, ACST-9:30)	1m 23s
rust_versions (ubuntu-latest, nightly)	1m 21s
timezones (ubuntu-latest, EST4)	1m 23s
timezones (ubuntu-latest, UTC0)	1m 20s
timezones (ubuntu-latest, Asia/Katmandu)	1m 18s
check-docs	21s
timezones (macos-latest, ACST-9:30)	1m 3s
timezones (macos-latest, EST4)	1m 8s
timezones (macos-latest, UTC0)	1m 3s
cross-targets (x86_64-sun-solaris)	1m 13s
timezones (macos-latest, Asia/Katmandu)	1m 1s
timezones (windows-latest, ACST-9:30)	2m 33s
timezones (windows-latest, EST4)	3m 17s
cross-tests (ubuntu-latest)	1m 55s
timezones (windows-latest, UTC0)	2m 39s
timezones (windows-latest, Asia/Katmandu)	2m 1s
test_wasi (ubuntu-latest, wasm32-wasi)	21s
test_wasm (ubuntu-latest)	23s
	57m 13s

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